Recently, as digital technologies have been advancing even farther, various kinds of electronic devices for capturing and processing image information digitally, including digital cameras and camcorders, have had their performance further enhanced in the fields of mobile telecommunications and other IT-based consumer electronics. And the higher the performance of those cameras, camcorders, and other electronic devices, the more and more essential it is to reduce the weight, thickness and cost of lenses and other optical members or systems to be used in those devices.
If a condenser lens for use in the optical system of such a camera or camcorder is implemented as a diffractive optical element, then there is no need to use multiple aspheric lenses with a complex surface shape. That is why by providing a diffraction grating on the surface of a lens body, the condenser lens, and eventually the overall optical system, can have their weight and thickness reduced. Furthermore, even a condenser lens for optical discs to be used in a broad wavelength range (e.g., in the visible radiation range of approximately 400-700 nm) can also be a single diffractive optical element. That is why just by adopting a diffractive optical element, white light can be condensed efficiently and the optical system can also be simplified and have its weight and cost reduced.
Meanwhile, if white light or any other light falling within a broad wavelength range impinges on such a lens for imaging, unwanted diffracted light could be produced, a flare or a ghost could debase the image quality, or an MTF (modulation transfer function) characteristic could deteriorate. To avoid such deterioration in characteristic, according to a well known technique, the surface of a lens body with a diffraction grating is coated with a layer of an optical material that has a different refractive index and a different dispersion of refractive indices from the lens body. Such a coating layer can be formed by either applying the optical material onto the surface of the lens body or bonding a film of the optical material onto the surface of the lens body.
In a diffractive optical element that has such a coating layer on the lens body, a diffraction grating depth d, at which the first-order diffraction efficiency becomes 100%, is theoretically given by the following Equation (1):
                    d        =                  λ                                                                n                ⁢                                                                  ⁢                1                ⁢                                  (                  λ                  )                                            -                              n                ⁢                                                                  ⁢                2                ⁢                                  (                  λ                  )                                                                                                    Equation        ⁢                                  ⁢                  (          1          )                    where n1(λ) is the refractive index of the material of the lens body and n2(λ) is the refractive index of the material of the coating layer. Both of these indices are a function of the wavelength λ.
If the right side of Equation (1) becomes constant in the wavelength range used, then the diffraction efficiency will no longer have wavelength dependence in that wavelength range, theoretically speaking. That is why if the lens body and the coating layer of a diffractive optical element are a combination of a material with a high refractive index and low wavelength dispersion and a material with a low refractive index and high wavelength dispersion, then the wavelength dependence of the diffraction efficiency can be reduced. As a result, a condenser lens that can condense even white light efficiently enough can be implemented as a diffractive optical element.
In such a diffractive optical element in which a number of optical material layers are stacked one upon the other and a relief pattern is provided in their interface, a configuration for reducing the wavelength dependence of diffraction efficiency and effectively preventing the occurrence of a flare due to color unevenness or light of unnecessary orders was proposed in Patent Document No. 1, for example. According to such an example, the lens body is made of optical glass as a high refractive index, low wavelength dispersion material, while the coating layer is made of either optical glass or an optical material of resin as a low refractive index, high wavelength dispersion material.
As for an optical system for an optical pickup for optical discs, it was proposed that a diffractive optical element that achieves high optical efficiency in the vicinity of each of 400 nm, 650 nm and 780 nm be used as an objective lens (see Patent Document No. 2, for example). In such an example, the lens body is made of optical glass as a high refractive index, low wavelength dispersion material, while the coating layer is made of an optical material of resin as a low refractive index, high wavelength dispersion material. In the interface between the lens body and the coating layer, a phase structure with concentric ring steps is arranged. The diffractive optical element with such a configuration realizes an objective lens that achieves high optical efficiency in each of the three wavelength ranges mentioned above.
In the conventional diffractive optical element that can condense white light efficiently (which will be referred to herein as a “white diffractive optical element”), a diffraction grating is formed by a compaction process on the aspheric surface of the lens body that is made of optical glass with a high refractive index and low wavelength dispersion, and then the surface is coated with a film of resin with a low refractive index and high wavelength dispersion. In such a white diffractive optical element thus formed, the diffraction grating itself has positive condensing power and the lens body with a high refractive index is coated with a resin layer with low wavelength dispersion. That is why a cross section of the diffraction grating has blazed steps, which define a decreasing function that steps down outward from the optical axis.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 9-127321    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2006-12394